In a semiconductor or liquid crystal manufacturing process, a peripheral portion or back surface of a substrate comes into mechanical contact with a hand of a transfer robot. When the hand of the transfer robot comes into contact with, for example, a thin film deposited on a side surface portion of the substrate, the thin film peels off, resulting in occurrence of particles. These minute particles bring an electrical trouble to a highly integrated device, and cause serious yield reduction. In addition, a conductive film which is formed so as to creep around the peripheral portion of the back surface of the substrate disturbs electrostatic adsorption of the substrate, or generates a DC current penetrating vertically through the substrate, thereby causing damage to elements in a subsequent process.
Selective removal of a film on the periphery of a substrate is considered as a method of preventing particles occurring due to mechanical contact with the peripheral portion of the substrate. For example, reaction gas is made to flow to the peripheral portion of the substrate, and high-frequency power is applied to the electrode provided above the peripheral portion to generate plasma around the peripheral portion. In this method, an extremely small interval between the electrodes is set in order to prevent diffusion of gas directed to the center of the substrate and wraparound of plasma. An ignition voltage of plasma is represented by a function of the product p*d of the internal pressure p of a reactor and the electrode interval d according to the Paschen's law. Since the ignition voltage of plasma is proportional to 1/d, the ignition voltage increases when d is extremely small, so that it is difficult to obtain uniform and stable plasma. Therefore, film formation or film removal at the peripheral portion of the substrate is non-uniform, which adversely affects the yield.